1. Field of the Invention
The present invention relates to a color conversion processing technique applied to color image equipment such as an electronic camera, a video camera, a scanner, a printer or the like.
2. Description of the Related Art
In general, color image equipment includes a color conversion circuit for carrying out color conversion processing (which may contain conversion of a color system or the like) in conformity with an input/output characteristic inherent to the color image equipment in order to provide an image signal with excellent color reproducibility.
The color conversion circuit is mainly classified into a look-up table circuit which is frequently used for a scanner, a printer or the like, and a matrix circuit which is frequently used for an electronic camera, a video camera or the like.
Of these circuits, the look-up table circuit stores pixel signals (R′, G′, B′) achieved after pixel signals (R, G, B) of each color are subjected to color conversion, and thus it can perform not only linear (first order) color conversion, but also higher-order color conversion. In such color conversion, complicated color conversion characteristics can be set in conformity with input/output characteristics of image equipment.
Therefore, according to the look-up table circuit, high color reproducibility can be provided to pixel signals, and all pictures of respective colors can be reproduced with proper colors. However, it has a drawback in that the amount of information to be stored is large, resulting in increase in circuit scale.
On the other hand, the matrix circuit can perform only linear color conversion because it merely stores a common conversion expression (normally, linear transformation matrix of 3×3) to pixel signals (R, G, B) of each color.
Therefore, it is impossible for the matrix circuit to provide high reproducibility to pixel signals, and pictures of all colors cannot be necessarily reproduced with proper colors. However, it has an advantage that the amount of information to be stored is small and an operation is simple, so that the circuit scale thereof can be suppressed to a small one.
With respect to the matrix circuit, it may be expected that the order of the color conversion be increased so that a color conversion characteristic is set like a curved line. In this case, however, for example when the order of the color conversion is incremented by 1 so that the curved line is set to a second-order curved line, the transformation matrix is scaled up to 3×9, and also it is required to generate vectors (R2, G2, B2, RG, GB, BR, R, G, B) adaptive to the transformation matrix. Therefore, the frequency of multiplication and division required for the whole color conversion is remarkably increased to +24, for example. Accordingly, in order to set the color conversion characteristic to a high-order curved line like a third-order or fourth-order curved line, it is needless to say that the circuit scale is remarkably increased.
Therefore, as in the case of the matrix circuit disclosed in Japanese Unexamined Patent Application Publication No. Hei 6-6587 or Japanese Unexamined Patent Application Publication No. Hei 5-300367, it has been attempted that plural kinds of coefficient groups of the transformation matrix are prepared in advance while the conversion order is kept to 1, and also these coefficient groups are selectively used in accordance with a color range to which a pixel signal belongs, whereby the color conversion characteristic can be approached to a proper curved line without increasing the circuit scale greatly.
The inventor of this application has found a problem that color conversion processing is sensitively varied by noise contained in a pixel signal and thus the color conversion processing is carried out so that the noise is conspicuous in the above-described related art.
Furthermore, since the color conversion characteristic is varied every pixel in the above-described related art, many places at which the color conversion characteristic is greatly varied occur on a screen. Therefore, there has been also concerned a problem that steps in the color conversion characteristic cause false signals (noise) and thus they are conspicuous.